High-throughput lithography systems are advantageous in the manufacture of many electronic and opto-electronic products, which require fabrication of millions of microscopic structures on a single large substrate. Such structures may be in the form of active devices, such as the transistors in an electronic display or in a semiconductor integrated circuit, or may be in the form of passive patterns, such as the metal interconnect network in a multichip packaging module or in a printed circuit board. The large substrate can be a display panel, a silicon wafer, or a board. The pattern feature sizes in these diverse products, which shall be referred to as electronic modules, range from sub-micron for semiconductor chips to multi-microns for displays and packaging products. The substrate size requirements vary from a few square centimeters for small modules to a few square feet for large displays.
A critical and common factor in the above applications is the need for a large-area patterning system that is capable of providing the required resolution over the entire substrate with high processing throughput. The patterning technology used determines not only the ultimate performance of the product (e.g., pixel density in a display, minimum device geometry in a chip, or interconnect density in a packaging module), but also the economics of the entire manufacturing process through such key factors as throughput and yield.
Conventional lithographic techniques that use masking technology involve a multi-step process which includes the photographic definition of patterns on optical masks which are used to impart these patterns onto the surfaces of individual substrates. The major exposure technologies used currently in patterning of electronic modules can be classified into three general categories:
(1) contact printing systems; PA1 (2) conventional and step-and-repeat projection systems; and PA1 (3) focused-beam laser direct-writing systems. Each of these will be briefly described below. PA1 1. Seamless scanning technology, developed by Anvik Corporation; PA1 2. Deformable micromirror device (DMD) technology, developed by Texas Instruments, Inc.; and PA1 3. Liquid crystal light valve technology, part of a vast field of technology and developed by numerous researchers.
(1) Contact Printing Systems
A contact printer for substrate exposure consists of a fixture to align and hold the board (i.e., the substrate) in contact with the mask, which is then illuminated with high-intensity light to transfer the mask image to the board. Systems that can handle boards as large as 610 mm.times.915 mm (24".times.36") are commercially available. A wide range of resolution capabilities is available in contact printers for different applications--from below a micron for semiconductor device fabrication to roughly 100 microns and larger for printed circuit board applications.
A desirable feature of contact printing systems is high throughput; however, the required use of contact masks contains a number of disadvantages. The process of designing and constructing a mask places a significant drag on the time required to build electronic module prototypes. The process of fabricating an electronic module involves the imaging of different layers and requires a different mask for each layer. The time required for switching and aligning masks as well as the expense of maintaining an array of masks for the production of a single electronic module represents a significant fraction of the cost of integrated circuit (IC) manufacturing. Eliminating the need for masks would reduce the long development time and minimize the high costs associated with IC production.
Furthermore, contact printing requires that, during exposure, the film or glass mask and the resist-coated board be brought in contact or near-contact. Good contact is difficult to produce over a large area. Poor contact, or a proximity gap, results in limited resolution. Frequent contact between the mask and the board causes generation of defects on the board which results in lower yields and causes reduction of the mask life, which leads to higher overall costs. In addition, variations in the gap cause feature size errors.
For certain electronic modules, such as flat panel displays (FPDs), the module size can be up to several square feet, requiring masks which are just as large. The technology to manufacture the masks themselves is a real impediment to manufacturing large-area FPDs. Eliminating the dependency on masks would sidestep this barrier; easing the difficulty of making the mask itself would be a great step forward.
(2) Projection Printing Systems
A wide variety of projection imaging systems are routinely used in fabrication of various electronic modules. Typically, a projection lens with a 1:1 magnification is used for imaging the mask pattern on the board. The illumination system uses a 1-2 kW mercury-xenon arc lamp, a heat-filtering mirror that filters away wavelengths in the visible and infrared regions, and a condenser to direct the radiation to the mask. All projection printing systems suffer from the limitation that there exists a trade-off between the resolution and the maximum image field size of the projection lens. For example, whereas 25 .mu.m resolution can be obtained over approximately a 100 cm square field, the imaging area for 1 .mu.m resolution must be limited to a field diameter no larger than 1-2 cm. For larger areas the total imaging field must be broken up into segments which then must be imaged one at a time in a step-and-repeat fashion, thereby limiting the available throughput. Most step-and-repeat systems use reduction imaging, typically with a 2:1, 5:1 or 10:1 ratio. Generally, systems with larger reduction ratios provide higher resolution, but also lower throughput.
Projection printing also requires the use of masks. As described above for contact printing systems, masking technology leads to many problems: use of masks does not allow for rapid prototyping of electronic modules; a different mask is required for subsequent layers of an electronic module, adding considerably to the expense of manufacturing; and the production capability of large-area masks does not meet all the current industry requirements for precision and accuracy at low cost and on fast schedules.
In addition, conventional projection imaging systems, due to fundamental lens-design considerations governing their performance, are forced to make a trade-off between resolution and image field size. This trade-off necessitates step-and-repeat imaging, in which significantly lower throughputs are obtained than by full-field contact printing. Lower throughputs result because each step involves the operations of load, unload, align, settle and focus. Step-and-repeat imaging also leads to higher costs due to the requirement of several masks and the errors introduced in stitching the different fields together.
The overlap or gap errors due to stitching the different fields together, usually referred to as "tiling," can be eliminated by complementary overlapping polygonal scans as described by a coinventor in U.S. Pat. No. 4,924,257, K. Jain, Scan And Repeat High Resolution Projection Lithography System, May 8, 1990.
U.S. Pat. No. 5,477,304, K. Nichi, Projection Exposure Apparatus, issued Dec. 19, 1995, uses a rectangular scanning aperture with a variable field stop, and uses orthogonally-moving mask stage and substrate stage to form a number of varying rectangles, in a tile pattern on a circular wafer.
Other systems use extensive shutter and blind components to form a useful exposure aperture, but still require a mask. See, as examples, U.S. Pat. No. 5,477,410, Projection Exposure Apparatus, issued Dec. 5, 1995, Nishi, and U.S. Pat. No. 5,227,839, Small Field Scanner, issued Jul. 13, 1993, Allen.
(3) Focused-Beam Direct-Writing Systems
A focused-beam direct-writing system uses a blue or UV laser in a raster scanning fashion to expose all the pixels, one at a time, on the substrate. The laser beam is focused on to the resist-coated board to the desired spot size. The focused spot is moved across the board in one dimension (say, along the y-axis) with a motor-driven scanning mirror. In conjunction, the stage holding the board is translated in the orthogonal dimension (x-axis) with a high-precision stepping motor. Simultaneously, the laser beam is modulated (typically, acousto-optically) to be either directed to the desired location on the board or deflected away. Thus, by driving the modulator and the two motors with appropriately processed pattern computer aided design (CAD) data, the entire board can be directly patterned. Of the many systems currently available, the offered resolution varies from 13-25 .mu.m for printed circuit board patterning to under a micron for systems designed for semiconductor applications. Since transfer of the pattern information in a scanning-spot direct-write tool takes place in a slow, bit-by-bit serial mode, typical processing times for such systems can range from 2 minutes to several hours per sq. ft., depending upon the resolution and the complexity of the pattern data.
Thus, although direct-write systems avoid the problems associated with using masks, they are extremely slow because transfer of the pattern information takes place in a bit-by-bit serial mode. Due to the large number of pixels (.about.10.sup.8 -10.sup.10) that must be written on an electronic module, typical processing times with such systems are unacceptably long for cost-effective volume manufacturing. Direct-write systems, therefore, are best suited for applications such as mask fabrication and prototyping.
From the above descriptions, it is clear that existing technologies for microelectronic patterning suffer from critical limitations. Thus there exists a need for a patterning system with the throughput of contact printers, the high resolution available from projection imaging, and the maskless feature of direct writing, without any of the disadvantages described above. The invention disclosed herein delivers all of the above performance and cost features in a patterning system, namely: a system which is maskless, has large-area capability, and images with high resolution, high throughput, and high yield.
The preferred embodiments of the invention rely on technologies such as the following:
These technologies are described below.
Seamless Scanning Technology
Seamless scanning patterning technology, invented by K. Jain, achieves the high-resolution, large-area, and high-throughput capabilities by a novel "scan-and-repeat" exposure mechanism. Exposure of arbitrarily large image fields, while maintaining the desired resolution, is made possible by a "seamless" scanning technique using partially overlapping complementary polygonal scans.
The following U.S. Patents are useful for the understanding of the seamless scanning technology applied to this invention:
U.S. Pat. No. 4,924,257, Scan and Repeat High Resolution Lithography System, K. Jain, May 8, 1990;
U.S. Pat. No. 5, 285,236, Large-Area, High-Throughput, High-Resolution Projection Imaging System, K. Jain, Feb. 8, 1994; and
U.S. Pat. No. 5,059,013, Illumination System to Produce Self-Luminous Light Beam of Desired Cross-Section, Uniform Intensity and Desired Numerical Aperture, K. Jain, Oct. 22, 1991.
Deformable Micromirror Device Technology
The Deformable Micromirror Device (DMD) is an opto-mechanical system which acts as a spatial light modulator that works in the reflective mode. The device consists of an array of hinged micromirrors which fit on a chip, each micromirror having the capability of tilting in two different rotations about an axis. When the micromirrors tilt in one direction, they may reflect radiation through an optical system, for imaging; thus these micromirrors are described as turned "on." Micromirrors that tilt in the other direction reflect radiation so that it does not pass through the optical system. These mirrors are turned "off." The set of micromirror devices arranged in an array, then, is the object of an imaging system where the "on" mirrors form the bright pixels of the object while the "off" mirrors form the dark pixels.
The following references are useful for the understanding of Deformable Micromirror Device (DMD) technology:
Hornbeck, Larry J., SPIE Critical Reviews Series, v. 1150, 1989, p. 86.
Younse, Jack M., IEEE Spectrum, November, 1993, p. 27.
Liquid Crystal Light Valve Technology
There are several systems that are capable of producing images utilizing deformable micromirror devices, but none provide a seamless pattern on a large substrate.
U.S. Pat. No. 5,504,629, Optical Projection System With A Novel Lens System, issued Apr. 2, 1996, D. Lim, shows a deformable micromirror device used to produce an image. However, Lim equips each individual micromirror with a baffle and microlens to vary the intensity of each image point produced.
Another application of deformable micromirror devices is shown in U.S. Pat. No. 5,452,024, DMD Display System, issued Sep. 19, 1995, J. Sampsell. Sampsell shows deformable micromirror devices used to produce an image for High Density Television (HDTV). Intensity is controlled by mirror ON-time, not by successive overlapping exposures. There is no scanning ability and no system for image error correction.
U.S. Pat. No. 5,105,369, Printing System Exposure Module Alignment Method and Apparatus of Manufacture, issued Apr. 14, 1992, W. Nelson, discloses a deformable micromirror device system that has scanning capability. Nelson discloses the use of a deformable micromirror device to produce an inverted image on a printing drum. Nelson shows no error correction, and the scanning is limited to one direction, the width of the deformable micromirror device array.
U.S. Pat. No. 5,296,891, Illumination Device, issued Mar. 22, 1994, H. Vogt et al., discloses an imaging system using a variable diffraction surface light modulator to provide a pattern, and using a schlieren lens system to project the pattern and eliminate non-pattern light. Vogt et al. requires a mirror (17) to block undiffracted light, and, if mirror (17) is partly reflecting, a shutter placed in the beam path to filter out the zeroth-order diffraction light (undiffracted light) which is reflected from the non-pattern areas of the surface light modulator.
A Liquid Crystal Light Valve (LCLV) is an electro-optic device which operates as a spatial light modulator in transmissive mode. It is programmable and has an array of pixels which can be directly addressed by a control system. Because the pixels are capable of switching from opaque to transmissive, they can be programmed to display desired images on the array itself. The LCLV thus can be used in transmissive mode to transmit images through an optical system.
U.S. Pat. No. 4,653,860, Programmable Mask or Reticle With Opaque Portions on Electrodes, issued Mar. 31, 1997, J. Hendrix, and Statutory Invention Register Disclosure H152, Method and System for High Speed Photolithography, B. Geil, Apr. 2, 1996, disclose using a liquid crystal as a shutter. Neither Hendrix nor Geil has scanning capabilities or complementary overlap for tiling error elimination.
There are exposure systems that are capable of scanning, but with limitations.
U.S. Pat. No. 5,045,419, Pattern Exposure/Transfer Method and Pattern Exposure/Transfer Mask Apparatus, K. Okumara, Sep. 3, 1991, discloses a system that utilizes collimated light and a liquid crystal mask. There is no error correction or overlap provided for and more complex patterning requires a plurality of liquid crystal masks.
U.S. Pat. No. 4,675,702, Photoplotter Using a Light Valve Device and Process for Exposing Graphics, issued Jun. 23, 1987, H. Gerber, describes a system wherein an image is exposed by varying the transmisivity of the liquid crystal light valve. Intensity is controlled by the time a valve remains open. Although there is some scanning capability, there are no provisions for seamless scanning or error correction.
U.S. Pat. No. 5,448,395, Non-mechanical Step Scanner for Electro-Optical Sensors, issued Sep. 5, 1995, M. Lopez discloses another method of using a liquid crystal light valve, using mechanical shutters. Scanning is accomplished by sequentially opening shutters, creating a mosaic of an image. There is no overlap capability; therefore there is no error correction and there may be seams in the final image.
There are a number of known techniques for distributing and combining laser pulses. See, as examples, U.S. Pat. No. 3,924,937, Method and Apparatus for S Sequentially Combining Pulsed Beams of Radiation, issued Dec. 9, 1975, J. Munroe at al; U.S. Pat. No. 4,073,572, System for Increasing Laser Pulse Rate with Beam Splitters, issued Feb. 14, 1978, K. Avicola; U.S. Pat. No. 4,283,116, Beam Combiner, issued Aug. 11, 1981, J. Weis. Munroe et al., discloses a technique for combining radiation pulses in plural beams to provide increased repetition rate, with optical correction for angular motion. Avicola discloses a technique for increasing laser pulse rate using beam splitters. Weis discloses an optical pulse distributor.
One embodiment of the invention disclosed integrates the seamless scanning system with Deformable Micromirror Device (DMD) technology, while another embodiment uses the seamless scanning system with Liquid Crystal Light Valve (LCLV) technology. The need for conventional masks is thereby eliminated. Furthermore, the invention enables high processing throughputs to be achieved, while maintaining high resolution over arbitrarily large image fields.
Disclosure of Invention
The invention is a maskless patterning system which is capable of imaging electronic modules by delivering high resolution over a large image field, with high exposure throughput. It is the object of the invention to eliminate the need for masks in the imaging of patterns on electronic modules, through the use of seamless scanning and a spatial light modulator that is directly addressed by a control system. Another object of the invention is to retain choice of light source, including laser and halogen lamp, and choice of spatial light modulator, including a deformable micromirror device (DMD) and an array of liquid crystal light valves (LCLVs). A seamless lithography system is incorporated here to achieve high resolution, large area, and high throughput by a scan-and-repeat exposure mechanism. Another object of the invention is to enable patterning of substrate panels that are several times greater in size than is currently possible. For a given panel size, processing throughput is increased substantially, at the same time that high resolution is maintained over the entire substrate. A feature of the invention is to achieve seamless scanning through electronically programming the spatial light modulator. The DMD or LCLV array is configured to reflect or transmit an image with a particular intensity profile, part of which may be integrated with the intensity profile of an overlapping, adjacent scan. Another feature of the invention is that the entire spatial light modulator may be illuminated so as to allow the modulator itself to generate the intensity profile characteristic of a scanning hexagonal field. An advantage of the invention is that it corrects for image reversal with the stream of data that is loaded onto the spatial light modulator, thus obviating the need for image reversing optics. Another advantage of the invention is its versatility, which enables it to be used in the production of several high-volume electronic products. A further advantage of the invention is that it eliminates the shortcomings of conventional contact, projection, and direct-writing systems.